1) Field of the Invention
This invention relates to an over-flow tank used in an apparatus for washing semiconductor wafers.
2) Description of the Prior Art
In a commonly accepted technique, a semiconductor wafer washing apparatus includes a plurality of washing tanks disposed in series which include a chemical tank containing liquid mixed with chemicals for initial washing, intermediate washing tanks, and a final washing tank containing pure water. Each of the washing tanks is surrounded by an over-flow tank so that the over-flow tank receives liquid over-flown from the washing tank and the liquid circulates through a pump and filter. Such a known washing apparatus is described in U.S. Pat. No. 4,520,834 to Dicicco. Although the known apparatus includes washing tanks and a frame for supporting and receiving the washing tanks, the top portions of the tanks and the frame are nearly the same in height. Accordingly, clean air introduced downward toward to the respective tanks will not be guided with any tank portions and thus clean air will scatter.
Also, a prior art over-flow tank provided with pipes and/or devices such as sensors, as shown in FIGS. 4 and 5, has its top margin of the level nearly identical to that of the washing tank located therein. Thus, one side wall of such over-flow tank is attached with an attachment plate with bolts or setscrews, and pipes for feeding or withdrawing chemicals and/or devices such as temperature sensor and level sensor are mounted on the attachment plate. Such a conventional over-flow tank has some drawbacks and problems as will be described below.
Since the conventional tank must be provided with the attachment plate on a side wall thereof, the attachment of the piece plate necessarily forms a stepped portion defining an upward-facing planar surface. This planar surface will undesirably tend to accumulate dust, thus lending to staining of the wafers. On the other hand, such washing apparatus is usually arranged in a clean room as seen in FIG. 6 and in the room each washing tanks are fed with clean air sent downwards from the ceiling through a filter and also through an opening formed in a cover. And, clean air flows over the washing tanks into a suction duct. Then, clean air sent on the washing tanks scatters, so that vapor formed in a tank included in the chemical tank row will flow transversely, that is, in crossing direction to the duct-oriented normal direction. Thus, the vapor will enter the final washing tank belonging to the next tank row. This influences the upper portion of wafers in the final washing tank, and will cause stain film on the wafer upper portion.